Résumé
Background: Sexual selection has initially been thought to occur
exclusively at the precopulatory stage in terms of contests among males
and female mate choice, but research over the last four decades revealed
that it often continues after copulation through sperm competition and
cryptic female choice. However, studying these postcopulatory processes
remains challenging because they occur internally and therefore are often
difficult to observe. In the transparent free-living flatworm Macrostomum
lignano, a recently established transgenic line that expresses green
fluorescent protein (GFP) in all cell types, including sperm, offers a
unique opportunity to non-invasively visualise and quantify the sperm of a
GFP-expressing donor inside the reproductive tract of wild-type recipients
in vivo. We here test several aspects of the reproductive performance of
the transgenic individuals and the accuracy of the techniques involved in
assessing the GFP-expressing worms and their sperm. We then show the
usefulness of these methods in a study on sperm displacement. Results:
GFP-expressing worms do not differ from wild-type worms in terms of
morphology, mating rate and reproductive success. In addition, we show
that the GFP signal is reliably and unequivocally expressed by all
GFP-expressing individuals observed under epifluorescence illumination.
However, the intensity of the GFP signal emitted by sperm of GFP
expressing donors can vary (which we show to be at least in part due to
sperm ageing) and the GFP marker is inherited according to Mendel’s laws
in most, but not all, of the individuals. Nevertheless, we argue these two
issues can be addressed with an appropriate experimental design. Finally,
we demonstrate the value of the GFP-techniques by comparing the number of
GFP-expressing sperm in a wild-type recipient before and after mating with
a competing sperm donor, providing clear experimental evidence for sperm
displacement in M. lignano. This result suggests that sperm donors can
displace previously stored sperm and replace it with their own.
Conclusion: The availability of the GFP-techniques in a transparent
organism provide unique opportunities to visualise and quantify internal
processes in the female reproductive tract after mating, which opens new
avenues in the study of sexual selection.